Characterization of an induced morphological defense in the Eastern Oyster Crassostrea virginica

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Abstract

To reduce their risk of consumption, many prey species use plastic defenses that are employed in situations where predators pose high risk of injury or death. By modifying prey traits, predators often exert strong influence on community structure and function. These nonconsumptive effects can be stronger than effects from prey consumption, but have only become the focus of study in recent decades. Further, they are often more complicated than consumptive effects because they can be influenced by characteristics of the predator, the prey, the response type, and the environment. In order to fully understand the effects of predator species on prey organisms, prey responses must be fully characterized and understood in an ecological context.
Eastern oysters Crassostrea virginica were selected as a model organism to investigate the costs, benefits, and mechanisms of predator-induced changes in prey traits. Oysters are an ecologically and economically valuable species, known to increase their shell weight and strength in response to crab predators. In this study, oyster responses were investigated by 1) fully characterizing the mechanism and cost of oyster responses to predation risk, and determining the effect of 2) risk cue type (predator versus prey), 3) an ecological prey characteristic (size), 4) an ecological predator characteristic (diet), and 5) an environmental characteristic (food availability) on defense induction.
In response to risk, oysters increased calcium carbonate production to rapidly reach a size refuge from predators. This led to an inverse relationship between shell thickness and shell density. Although not significant, there was a trend for reduced gonad investment in oysters which had thick, low density shells. As this study provides a conservative estimate of defense costs, this suggests oysters may experience reductions in reproduction under conditions of high predation risk which may be exacerbated under conditions of ocean acidification.
Oysters responded to all tested cues indicative of risk, but responses were less intense when oysters were exposed to cues from injured prey organisms or to predators which were food deprived or fed aged oyster tissue. In addition, oyster responses were inversely related to oyster size. Oysters responded to predators regardless of resource availability, despite reduced shell metrics in response to limited food under control conditions.
There is a diverse array of factors which influence oyster defenses and which may be altered by anthropogenic disturbance. If oyster defenses reduce investment in reproduction, this could have important long-term consequences for oyster populations and oyster reef communities. Therefore, understanding factors which influence prey responses allows us to understand conditions which promote or attenuate nonconsumptive predator effects. And understanding the costs of these effects for organisms allows us to predict the effect of disturbances on community structure and function.